The liquid crystal display device is used as an image display device of small space-saving and of low power consumption, and a field of application thereof is widened year by year, and mainly TN mode is used widely. In this mode, since the liquid crystal rises up against the substrate at the black display, birefringence due to such the liquid-crystal molecules generates when being observed in an oblique direction, and light leakage occurs. For this problem, liquid-crystal cells are optically compensated by using a film formed of hybrid-aligned liquid-crystal molecules, and such mode for preventing the light leakage is put to practical use. However, it is extremely difficult to optically compensate liquid-crystal cells perfectly without causing problems even if liquid-crystal molecules are used, and problem arises in that contrast inversions generating at under areas of images cannot be avoided.
In order to solve the problem, a liquid crystal display device employing so-called in-plane switching (IPS) mode, in which the crosswise fields are applied to liquid crystal, or vertically aligned (VA) mode of vertically aligning the liquid crystal having a negative dielectric anisotropy and dividing the alignment by a protrusion formed in the panel or by a slit electrode, have been proposed and put into practical use. According to theses modes, demands for the liquid crystal display device which exhibit high brightness are rapidly increasing even in the market where a high quality image such as television is required.
Accordingly, small light leakage generating at opposing corners in an oblique incident direction at the black display, which has been heretofore not a problem, has elicited as a cause of lowering displaying-quality. Additionally, further improvements on optical compensation properties that exhibit high contrast and decrease changes in phase difference have been demanded for the optically-compensatory film.
As one of means to improve this color tone or viewing angle of black display, it has been also studied to dispose an optical compensatory material having birefringence between the liquid-crystal layer and the polarizing plate in an IPS mode.
A birefringent medium, in which the optical axes having activity of compensating for the increase or decrease in the retardation of the liquid crystal layer at the inclination are orthogonal to each other, disposed between the substrate and the polarizing plate so as to improve the color when a white or halftone display is directly viewed from the oblique direction, has been disclosed (See Japanese Unexamined Patent Application Publication No. 9-80424). In addition, there is proposed a method of using an optically-compensatory film comprising a styrene-based polymer or discotic liquid-crystal compound having a negative intrinsic birefringence (See Japanese Unexamined Patent Application Publication No. 10-54982, Japanese Unexamined Patent Application Publication No. 11-202323 and Japanese Unexamined Patent Application Publication No. 9-292522), a method of laminating a film in which the birefringence is positive and optical axes are inside the film, and a film in which the birefringence is positive and its optical axis is in a direction normal to the film, as an optically-compensatory film (See Japanese Unexamined Patent Application Publication No. 11-133408), a method of using a biaxial optical compensation sheet of which the retardation is half the wavelength (See Japanese Unexamined Patent Application Publication No. 11-305217), and a method of employing a film which has negative retardation as a protective film for a polarizing plate and providing an optical compensation layer which has positive retardation to a surface of the film (See Japanese Unexamined Patent Application Publication No. 10-307291).
Recently, there has been proposed an optically-compensatory film having a high retardation value which can be used in applications requiring optical anisotropic properties by using a cellulose acylate film. Since many of such films have high stretching magnification and a retardation regulator, the retardation can be controlled in a wide range. As a cellulose acylate film in which an optical axis is in a normal direction of the film, there has been proposed a method of cooling cellulose acylate which has low acyl substitution degree (See Japanese Unexamined Patent Application Publication No. 2005-120352).
In addition, as a means for optical compensation, an optically compensatory film having a negative retardation in the film thickness direction (Rth), in particular, a cellulose ester film which can be used as a protective film for polarizing plates, is being demanded.
In this regard, for example, JP-A No. 2005-120352 suggests a technology of preparing a cellulose acylate film having a negative Rth, by adequately selecting the conditions for preparation, such as the degree of substitution in cellulose acetate, dissolving method, and the like. Furthermore, JP-A No. 2005-99191 suggests a technology of reducing the retardation using compounds having a specific structure.